Method of manufacturing reflecting optical elements



July 6, 1948. 2,444,532

J. H. RICHARDSON METHOD OF MANUFACTURING REFLECTING OPTICAL ELEMENTS Filed llarch 7, 1946 NITED STAThg PATENT orrica METHOD Ob -MANUFACTURING REFLECT- .me OPTICAL ELEMENTS .J 011 11 H. Richardson, Needham, Mass., assignor to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware I The present invention relat'sgeh'erally proved methods of forming optical'elem j .Applica tion Marti 7, 1946, Serial No. 652,831

: 11.Claims. (01.18-59) I 2 to im-- ents as illustrated herein and adapted for use in a 'Schmidt projection system. The manufacture of precision glass optical elements is expensive and time consuming; The grinding and, polishing operations necessary to produce smooth optical surfaces are slow and tedious, and are not adaptable to mass production methods. It has heretofore been proposed to form precision optical elements," such as mirrors, by applying a polymerizable composition to a base member preferably formed of a mineral hydrate and having a surface thereon of a COD.-

' tour which approximates the contour of the finished optical surface of the element. The composition is polymerized on the base member in contact with a mold having a smooth surface thereon which conforms exactly optical to the 'contour of the optical surface of the finished optical element. The polymerizable composition is of such nature that it forms a'hard; optically "smooth homogeneous surface when polymerized in contact with the mold. Under some conditions, however, checks and cracks-1 appear upon the finished optical surface of'the' coat ment.

edele-- Such checks and cracks, in some 'cases,

are the result of strains set up in theqcoating because of the great difference in-athe lin lefiicients of expansion of the mineral hydrate or ear CO- der other conditions, the polymerizable: sition may penetrate excessively into the backing and thus destroy or prevent the comporplaster formation of a smooth finished optical surface.

One object of the present inventionis vide an improved method of makings-precision to'. prooptical elements whereby the above.mentioned diificulties will be overcome.

Tothis end, and

as illustrated, a partial polymer ofra polymerizable composition is applied to the'rmoldingsur- .face of a heated mold to form a coating thereon.

The mold, with the partially polymerizedecating thereon, is subsequently placed in a heated oven until the outer surface of the coating further polymerizes slightly. The coated moldis then removed from the oven anda plaster or other base is placed or formed on 'the' coated surface, and the combined base and mold placed in an oven and heated until the plastic coating is completely polymerized. r

Still further objects of ;the-' invention:

;are to "provide improved optical elements and methods of making the same. Other objects and advantages will in part ap- "pear and in part be pointed out in the course of ".3 the following descriptions of the invention, which "are given as nonlimiting examples in connection with the accompanying drawings in which: Figure 1 is a schematic view, in section, illusto the convex surface of a mold member;

Fig. 2 is a view, in section, of a mold member 'having a coating of partially polymerized mate- 'rial thereon; I

Fig. 3 is a sectional view illustrating one form of a backing member to which the coating may "be secured; I Fig. 4 is a view, in section, showing the relation between the mold and the backing member with a thin coating of a partially polymerized g0 material interposed therebetween;

Fig. 5 is a sectional view illustrating a finished *Optical element;

Fig. 6 is an enlarged fragmentary view, in sec- '--tion, of a portion of the backing member having "a completely polymerized coating thereonyand Fig. '7 is a sectional view illustrating a method 0f casting the plaster backing member. 7 i The optical element [0 shown in the drawings, as illustrated best in Fig. 5, is provided with a concave spherical reflecting surface l2. The

I? curvature of this surface however, may be para- "bolic, hyperbolic, or of any other desired contour f-or curvature. The present optical element I0 is particularly adapted for use as a reflector in a'Schmidt projection system, but it is apparent th'at it could also be used as a reflector for other purposes. These reflectors vary substantially in size and are usually from 6 to 14 inches in diameter, but' they may, in. some instances, be as 40 small as two inches in diameter and as large as six or more feet in diameter. fi Theoptical element '10 comprises a rigid base 4 "having a concave spherical surface [6 which rapproximates the contour of the finished reflect- -=.ing surface l2. The base member 14 is prefer- --ably formed of rigid material which is unaffected by theusual temperature changes to which the .i'optical element may be subjected. The base :member is preferably formed from material :which can be readily cast or molded with the sur- -face l6 thereon approximating the contour of the finished reflecting or optical surface [2 of .the element. Materials used for forming the --base. member are preferably mineral hydrates, oafsuchr forexample as plaster-of Paris, Portland trating one method of applying the fluid coating in place by a wrapping of scotch tape 28. This provides a substantially water-tight cavity or receptacle into which a suitable mixture of plaster of Paris or the like may be poured. After the plaster of Paris has set, the mold with the rigid backing thereon is placed in an oven and baked for a period of about 2 hours at a temperature of approximately 80 C. until the partial styrene polymer has been completely polymerized. The mold with the base l4 thereon is then removed from the oven, cooled, and the mold separated face of the element or as is preferable, the base member is cast directly on the coating formed on the mold member.

The mold it having a convex smooth optically finished surface 20 thereon is formed of glass or other suitable material which may be ground and polished to provide a smooth optically finished molding surface. The mold I 8 is coated with a partially polymerized plastic which may be styrene, orthochloro styrene, or other members of the styrene family. Other polymerizable resins may also be used, such as cyclohexyl methacrylate, methyl methacrylate, benzyl methacrylate .and copolymers and interpolymers thereof. It is to be expressly understood, however, that all materials are contemplated which may be readily hardened by heat, and which, when hardened in contact with the mold surface 20 form a coating which is hard and homogeneous and form an optically smooth finished surface.

The coating 22 may be applied to the mold by pouring a thin film of partially polymerized styrene over the convex surface thereof and allowing the surplus material to drain off. Preferably, however, the convex surface of the mold i8 is dipped into a bath 213 of partial polymer of styrene, as shown, for example, in Fig. l. The partial polymer preferably has a viscosity of approximately 10,000 centipoises. The temperature of the mold is approximately 80 0., but it is evident that the temperature of the mold may vary, d pending to some extent upon the temperature at which the partial polymer is polymerized. The temperature of the partial polymer can vary sub stantially, but is usually somewhere within the range of 23 to 80 C. After the convex surface 20 of the mold has been dipped in the bath 24 of the partial polymer, it is removed therefrom and placed with its convex side upwardly to permit the excess partial polymer to drain off, as shown in Fig. 2. With a partial polymer of styrene of the viscosity set forth above and with the temperature of the mold and the temperature of the polymer at about 80 C., the layer of plastic on the convex surface 20 will be approximately .005" thick. If a thicker coating is desired, the viscosity of the partial polymer may be increased by further polymerization or by cooling.

After the mold is covered, or coated, by either of the above methods, it is placed in an oven, heated to approximately 80 C. and baked for approximately minutes to further partially polymerize the styrene and until a thin skin or film is formed on the exposed surface of the polymerizable coating. After this baking operation has been completed, the mold with the coating 22 thereon is removed from the oven and permitted to cool. As shown in Fig. '7, the plaster base l4 may be formed by casting the base from a slurry of plaster of Paris or other similar hydrated material directly on the coated convex surface of the mold I8. A strip 26 of polyvinyl alcohol or Cellophane is wrapped around the upper cylindrical surface of the mold, l8 and is held from the plastic coated base I4 in the usual manner. After the optical element [0 has been suitably cooled, for example to room temperature, the optical surface l2 thereof is provided with a reflecting coating 30 which may be of silver and which may be deposited thereon in'any wellknown manner. The reflecting coating 30, however, may also be formed by depositing a thin layer of evaporated aluminum thereon. After the reflecting coating 30 has been suitably polished,

the optical element It) is ready for use.

In some cases it may be desirable to precast the rigid base member or support [5 with a concave surface [5 thereon approximating substantiall the contour of the finished optical surface of the element.

Such a precast base member or support 14 is placed on the convex coated surface of the mold in the manner shown in Fig. 4. The thin skin of film which is formed on the above mentioned partial polymerization of the coating prevents the styrene from penetrating into the base member M to any substantial extent, such as would prevent formation of a smooth optically finished surface on the coating 22 when the coating 22 is completely polymerized. After the mold l8 and the precast plaster base 14 are assembled, for example, as shown in Fig. 4, they are placed in an oven and baked for approximately 2 hours at a temperature of about C. to completely polymerize the styrene. After this baking process has been completed and the styrene completely polymerized, the mold and the optical elements are gradually cooled and the coated optical element I0 is separated from the mold in the usual manner.

By the use of either of the above mentioned processes, the coating 22 is polymerized and secured to the base l4 in a single operation. Because of the thin skin or film which is formed on the backing, the polymerizable composition does not appreciably penetrate into the backing, thus a coating is formed which is substantially uniform in thickness and which has a hard, smooth polished exposed face.

Since certain changes in the constructions and methods set forth which embody the invention, may be made without departing from its scope, it is intended that all matter contained in the above described specification, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. An improved method of manufacturing reflecting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface with a liquid partial polymer of a polymerizable material-,advanchig the polymerization-of the exposed surface of said liquid partial polymer coating to form a thin "flexible polymerized skin while retaining the remainder of the partial polymer in liquid condition, applying a porous backing to said polymerized skin, said porous backing belnga'materiarchemically inert ate and incapable'of dissolving said polymerized skin but which will adhere thereto,-and,completing the polymerization of said liquid partial polymer coating to cause the polymerized coating to be bonded to said backingfJ- jfi 2.. An"-1mproved methodof manufacturing. refl'ecting optical elementswhich'compriss.fir6vida" mold -member -having anoptically-lfinished -molding surface, -coating sald -optically finished molding suriaceyithm polymerizable liquid material, a.dvancing the polymerization of the exr posed surface of said polymerizable liquld coating to 'form athinflexible polymerized skin Whileum fl flngdo be bonded'to said ng- 1 retaining the remainder of said coating in liquid condition, applying a porous backing to said polymerized skin, said porous backing comprising a material chemically inert to and incapable of dissolving said polymerized skin but which will adhere thereto, and completing the polymerization of said liquid coating to cause the polymerized coating to be bonded to said backing.

3. An improved method of manufacturing reflecting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with styrene monomer, advancing the polymerization of said styrene monomer coating to form a thin skin of polymerized styrene while retaining the remainder of said coating in liquid condition, applying a porous backing to said polymerized styrene skin to provide a rigid support therefor, said porous backing comprising a material chemically inert to and incapable of dissolving said polymerized skin but which will adhere thereto, and completing the polymerization of said styrene coating to cause the polymerized coating to be bonded to said backing.

4. An improved method of manufacturing refleeting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with a liquid partial polymer of styrene, advancing the polymerization of said styrene coating to form a thin skin of polymerized styrene while retaining the remainder of said coating in liquid condition, applying a porous backing to said poLvmerized styrene skin to provide a rigid support therefor, said porous backing comprising a material chemically inert to and incapable of dissolving said polymerized skin but which will adhere thereto, and completing the polymerization of said partial polymer of styrene to cause the polymerized coating to be bonded to said backing.

5. An improved method of manufacturing reflecting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with a liquid partial polymer of styrene, advancing the polymerization of said styrene coating to form a thin skin of polymerized styrene while retaining the remainder of said coating in liquid condition, casting a porous backing on said polymerized styrene skin to provide a rigid support therefor, said porous backing comprising a material chemically inert to and incapable of dissolving said poly- 'filflliid biit'which will adhere theretmadd -completing the polymerization of said partial f-polymer coating-to cause the polymerized coating to be bonded'to said'ba'cking'.

2 8;-'An'imptov'ed method of manufactin-lng refie'cting optical elements which pro- -viding 'c; mold member" having an optically finilshed molding said optically fin- *ishedmolding surface withla polymerizable liquid the polymerization of the ex- B'iirface'of saidpolymerizable liquid coating -to form a thin 'fiexlble skin while re- -taining the 'reniainder-'of said coating in liquid -conditlonrcasting a p'ormis' backing on said polyrnerized akin'nsa'id porousbacking comprising -a -material chemically inert to and incapable of diseolvingsaid polymerizedskin but which will adhere and completlng the polymerization 'iofraaldsliquid coating to cause the polymerized 'll. improved of manufacturing refleeting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with a liquid partial polymer of styrene, advancing the polymerization of said styrene coating to form a thin skin of polymerized styrene while retaining the remainder of said coating in liquid condition, casting a slurry of mineral hydrate on said polymerized skin, said mineral hydrate being chemically inert to and incapable of dissolving said polymerized skin but being capable of adhering thereto, causing said mineral hydrate slurry to set to form a porous backing, and completing the polymerization of said partial polymer of tyrene to cause the polymerized coating to be bonded to said backing.

8. An improved method of manufacturing reflecting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with a liquid partial polymer of styrene, advancing the polymerization of said styrene coating to form a thin skin of polymerized styrene while retaining the remainder of said coating in liquid condition, casting a slurry of plaster of Paris and water on said polymerized skin, causing said plaster of Paris slurry to set to form a porous backing, and completing the polymerization of said partial polymer of styrene to cause the polymerized coating to be bonded to said backing.

9. An improved method of manufacturing reflecting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished molding surface of said mold member with a liduid partial polymer of styrene, advancing the go polymerization of said styrene coating to form a 7o merized coating to be bonded to said backing.

10. An improved method of manufacturing refleeting optical elements which comprises providing a mold member having an optically finished molding surface, coating said optically finished 1s molding surface with a polymerizable liquid mg.

. comprising a material chemically inert to and inpreformed plaster of Paris backing to said .par-

7 terial, advancing the polymerization of the ex- .tially polymerized coating, and completing the posed surface of said polymerizable liquid coatpolymerization of said partial polymer of styrene ing to form a thin flexible polymerized skin while to cause the polymerized coating to be bonded to retaining the remainder of said coating in liquid said plaster of Paris backing, condition, applying a porous preformed backing 5- JOHN H. RICHARDSON. to said polymerized skin, said porous backing REFERENCES CITED capable of dissolving sa d polymerized skin but i The following references are of record in the which will adhere thereto, and completingthe .m m Patent; 7

polymerization of said liquid coating to cause 18:210. I i TENTS polymerized coating to be bonded to said backing. sTTEs PA 11. An improved method of manufacturing re- Number Namev Date fl t n opti l l m nts whicncompris s P 1,094,828 51; aylesworth Apr. 28, 1914 viding a mold member-having an Optical y-m1- 42,313,985 Bradshaw Mar. 16, 1M3 ished molding surface, coating said Optically finy' 2,339,433 r Btaehle Jan, 18, 1944 ..ished molding surface with a liquid partial poly;F ;2,369,75B1.fz Sheldon Feb. 20, 1945 mer of styrene, advancing the polymerizationo 5.21:2, r. mm

. said-styrene coatingto form a thin skinof @013- i GN P 1 mm 1 me Great Britain in"... Apr. 5, 1938 merized styrene while retaining the remainder ,Nuniher oi said coating in liquid condition. applyingwa -482.836,,-1 

